Interhemispheric modulation of somatosensory receptive fields: evidence for plasticity in primary somatosensory cortex.

Extracellular recordings were made from single and multiple neurons in primary somatosensory cortex (area 3b) of macaque monkeys and flying foxes. When a small region of area 3b (or adjacent area 1) in the opposite hemisphere was cooled, thereby blocking activity that is normally transferred via the corpus callosum, larger receptive fields (RFs) were immediately unmasked for most neurons. RF expansion presumably reflects the expression of afferent inputs that are normally inhibited, suggesting that callosal inputs provide a source of tonic inhibition that contributes to the shaping of neuronal RFs. Quantitative analyses of single neuron responses revealed other effects that were consistent with a release from inhibition, such as increases in response magnitude to stimulation of points within the original RF and decreases in response latency. An unexpected finding was the reversal of these unmasking effects with extended periods of cooling: RFs returned to their original dimensions and within-field response magnitude decreased. In contrast to the initial effects, this reversal of disinhibition cannot be readily explained by an unmasking of previously unexpressed inputs. Any explanation for the reversal requires an increase in the efficacy of interneuron-mediated inhibition, and presumably occurs in response to ongoing, altered patterns of activity.